1. Field of the Invention
This invention relates to laser systems and more particularly to laser systems which produce light by frequency doubling or sum-frequency mixing.
2. Description of the Prior Art
Semiconductor diode lasers are of interest for a number of applications such as optical data storage, laser printing, and biochemical analysis. One example is the gallium-aluminum-arsenide (GaAlAs) diode laser which generates laser light in the near-infrared range (750-860 nm in wavelength). In optical data storage systems, the light from the laser diode is focused onto a spot on the optical disk in order to record each bit of data. The spot size is equal to approximately .lambda./(2*(N.A.)), where .lambda. is the wavelength of the light and (N.A.) is the numerical aperture of the focusing lens. In typical systems, the (N.A.) is approximately 0.5 and the resulting spot size is approximately 800 nm in diameter.
It is apparent that if the wavelength of the laser light can be cut in half, the diameter of the spot size will also be cut in half and the overall storage density on the optical disk may be quadrupled. Unfortunately, laser diodes that produce light in the blue wavelength range (430 nm in wavelength) are not available.
One technique to convert light to a higher frequency (shorter wavelength) is known as second harmonic generation (SHG). A laser beam at a first lower frequency is passed through a nonlinear crystal, such as potassium niobate (KNbO.sub.3), which produces a second harmonic laser beam (i.e., a beam at twice the frequency of the original laser beam which entered the nonlinear crystal). This SHG technique is discussed in articles by M. K. Chun, et al., Applied Physics Letters, Vol. 55, p. 218, July 17, 1989; and P. Gunter, et al., Applied Physics Letters, Vol. 35, p. 461, Sept. 15, 1979.
However, since the diode laser's output power is low, techniques to improve the second harmonic generation efficiency are required in order to produce a useful and efficient laser system.
One way to increase the efficiency of the SHG scheme is to place an optical resonator or cavity around the nonlinear crystal. The light is reflected back and forth through the crystal inside the resonator in order to generate a substantial amount of the blue light. This technique was originally proposed and demonstrated by Ashkin, et al., IEEE Journal of Quantum Electronics, Vol. QE-2, p. 109, 1966. Other examples include Goldberg, et al., Applied Physics Letters, Vol. 55, p. 218, 1989; and Baer, et al., Conference on Lasers and Electro-Optics, Paper THM5, 1989. Frequency doubling of GaAlAs diode lasers using a build-up cavity containing a nonlinear crystal such as potassium niobate (KNbO.sub.3) offers the potential for the design of simple, compact laser systems. For the build up to occur, the external cavity resonance frequency must match the diode laser frequency, and the prior art includes a variety of techniques for achieving this frequency matching (e.g., Dixon, et al., Optics Letters, Vol. 14, p. 731, 1989; R. W. Drever, et al., Applied Physics B, Vol. 31, p. 97, 1983; and W. J. Kozlovsky, et al., IEEE Journal of Quantum Electronics, Vol. 24, p. 913, 1988.)
Heretofore, the nonlinear crystal KNbO.sub.3 has been used for resonantly enhanced frequency doubling of GaAlAs laser diodes. Potassium niobate has a large nonlinear coefficient and sufficient birefringence for phasematching of second-harmonic generation at the wavelengths of GaAlAs laser diodes. However, this phasematching is very sensitive to the temperature of the crystal, and this temperature must be precisely controlled to maintain efficient second-harmonic generation.
Nonlinear crystals other than KNbO.sub.3 have been shown to have phasematching properties advantageous for generation of blue/green light by frequency upconversion of semiconductor laser diodes. In particular, potassium titanyl phosphate (KTiOPO.sub.4, KTP) can be used for second-harmonic generation of 990 nm strained-layer InGaAs laser diodes (e.g., W. P. Risk, et al. Applied Physics Letters, Vol. 55, No. 12, p. 1179, and pending U.S. patent application Ser. No. 07/570,251 filed Aug. 17, 1990 by Harder, et al.) and has been shown to have broad temperature tolerances in that application. Similarly, sum-frequency mixing in KTP (e.g., J. C. Baumert, et al. Applied Physics Letters, Vol. 51, p. 2192, 1987 and U.S. Pat. No. 4,791,631) of a wavelength &lt;994 nm with a wavelength &gt;994 nm supplied by a combination of GaAlAs and InGaAs lasers can be used to generate virtually any blue/green wavelength between 450 nm and 500 nm.
The nonlinear processes described above in KTP require the presence of two orthogonally polarized infrared lightwaves in order to be efficient. Such interactions are known as Type-II nonlinear interactions. This is in contrast to the case of second-harmonic generation in KNbO.sub.3 where only one polarization is required (Type-I nonlinear interaction). Hence, to enhance the efficiency of a Type-II nonlinear interaction requires that two lightwaves having orthogonal polarizations, at the same or different wavelengths, be simultaneously resonated.
Monolithic resonators have reflective surfaces which are integrally formed on the nonlinear crystals such that the resonating lightwaves never leave the nonlinear crystal. This is highly desirable for reasons of efficiency, stability, and compactness. The preferred configuration in a Type-I nonlinear process is a triangular ring resonator which has a three sided beampath.
The inventors have discovered that enhancement of a Type-II nonlinear process has never previously been demonstrated in a monolithic ring resonator, due to the phenomenon of bireflection. Bireflection causes the ring path to depend on polarization of the light wave, so that the two polarizations required for a Type-II interaction cannot be simultaneously resonated in a triangular three-mirror monolithic ring, such as that used for Type-I interactions. What is needed is a new ring resonator geometry that permits both of the waves required for the Type-II interaction to be simultaneously resonated along the same ring path.